High speed internal permanent magnet machine and method of manufacturing the same

ABSTRACT

An internal permanent magnet (IPM) machine is provided. The IPM machine includes a stator assembly and a stator core. The stator core also includes multiple stator teeth. The stator assembly is further configured with stator windings to generate a magnetic field when excited with alternating currents and extends along a longitudinal axis with an inner surface defining a cavity. The IPM machine also includes a rotor assembly and a rotor core. The rotor core is disposed inside the cavity and configured to rotate about the longitudinal axis. The rotor assembly further includes a shaft. The shaft further includes multiple protrusions alternately arranged relative to multiple bottom structures provided on the shaft. The rotor assembly also includes multiple stacks of laminations disposed on the protrusions and dovetailed circumferentially around the shaft. The rotor assembly further includes multiple permanent magnets for generating a magnetic field, which interacts with the stator magnetic field to produce torque. The permanent magnets are disposed between the stacks. The rotor assembly also includes multiple bottom wedges disposed on the bottom structures of the shaft and configured to hold the multiple stacks and the multiple permanent magnets.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

This invention was made with Government support under contract numberDE-FC26-07NT43122 awarded by U.S. Department of Energy. The Governmenthas certain rights in the invention.

BACKGROUND

The invention relates generally to an internal permanent magnet machineand more particularly, to a high-speed internal permanent magnet machinewith high power-density.

Internal Permanent Magnet (IPM) Machines such as IPM motors orgenerators have been widely used in a variety of applications includingaircraft, automobiles and industrial usage. However, IPM machines arecurrently the clear favorite in hybrid automotive applications.Therefore, a requirement for lightweight and high power density IPMmachines has resulted in the design of higher speed motors andgenerators to maximize the power to weight ratios. Hence, the trend isincreasing acceptance of IPM machines offering high machine speed, highpower density, reduced mass and cost. In a conventional IPM machine,multiple permanent magnets are embedded inside multiple laminations of arotor. The mechanical stresses in the rotor are concentrated in multiplebridges and center posts. For higher speed applications, the thicknessof the multiple bridges and center posts have to be increased forenhanced structural strength of the rotor and various other parts. Theincreased thickness leads to more magnet flux leakage into the multiplebridges and center posts, which leakage significantly reduces themachine power density, resulting in decreased efficiency of the machine.

Therefore, it is desirable to have a high-speed IPM machine with highpower density and efficiency.

BRIEF DESCRIPTION

In accordance with an embodiment of the invention, an IPM machine isprovided. The IPM machine includes a stator assembly and a stator core.The stator core also includes multiple stator teeth. The stator assemblyis further configured with stator windings to generate a stator magneticfield when excited with alternating currents and extends along alongitudinal axis with an inner surface defining a cavity. The IPMmachine also includes a rotor assembly and a rotor core. The rotor coreis disposed inside the cavity and configured to rotate about thelongitudinal axis. The rotor assembly further includes a shaft. Theshaft further includes multiple protrusions alternately arrangedrelative to multiple bottom structures provided on the shaft. The rotorassembly also includes multiple stacks of laminations disposed radiallyoutward on the protrusions and dovetailed circumferentially around theshaft. The rotor assembly further includes multiple permanent magnetsdisposed between the stacks. The permanent magnets are configured togenerate a magnetic field, which magnetic field interacts with thestator magnetic field to produce a torque. The rotor assembly alsoincludes multiple bottom wedges disposed radially outward on the bottomstructures of the shaft and configured to hold the multiple stacks tothe shaft.

In accordance with another embodiment, an IPM machine is provided. TheIPM machine includes a stator assembly and a stator core. The statorcore also includes multiple stator teeth. The stator assembly is furtherconfigured with stator windings to generate a stator magnetic field whenexcited with alternating currents and extends along a longitudinal axiswith an inner surface defining a cavity. The IPM machine also includes arotor assembly and a rotor core. The rotor core is disposed inside thecavity and is configured to rotate about the longitudinal axis. Therotor assembly further includes a shaft with multiple protrusions. Therotor assembly includes multiple stacks of laminations mounted inbetween the protrusions and dovetailed circumferentially around theshaft. The laminations further include multiple magnetic regions andnon-magnetic regions. The rotor assembly also includes multiplepermanent magnets disposed between the stacks and are configured togenerate a magnetic field that interacts with the stator magnetic fieldto produce a torque.

In accordance with an exemplary embodiment, a method for assembling anIPM machine is provided. The method includes providing a stator assemblyincluding a stator core with multiple stator teeth. The method includesproviding the stator assembly configured with stator windings togenerate a stator magnetic field when excited with alternating currents.The method further includes providing the stator assembly extendingalong a longitudinal axis with an inner surface defining a cavity. Themethod also includes providing a rotor assembly with a rotor coredisposed inside the cavity and is configured to rotate about thelongitudinal axis. The method includes providing a shaft with multipleprotrusions alternately arranged relative to multiple bottom structuresaround the shaft. The method further includes providing multiple stacksof laminations disposed on the protrusions and dovetailedcircumferentially around the shaft. The method also includes providingmultiple permanent magnets disposed between the stacks and configured togenerate a magnetic field that interacts with the stator magnetic fieldto produce a torque. The method further includes providing multiplebottom wedges disposed on the bottom structures of the shaft andconfigured to hold the multiple stacks of laminations and the multiplepermanent magnets. The method also includes providing multiple topwedges covering the outer periphery of the rotor assembly to positivelyrestrain the multiple permanent magnets.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a cross-sectional view of a rotor assembly of an IPM machinein accordance with an exemplary embodiment of the invention.

FIG. 2 is a cross-sectional view of a stator assembly of an IPM machinein accordance with an exemplary embodiment of the invention.

FIG. 3 is a semi-cross-sectional view illustrating the magnetic fluxlines of an IPM machine in accordance with an exemplary embodiment ofthe invention.

FIG. 4 is a partial cross-sectional view of a rotor assembly of an IPMmachine in accordance with an exemplary embodiment of the invention.

FIG. 5 is a semi-cross-sectional view of an IPM machine in accordancewith another embodiment of the invention.

FIG. 6 is a semi-cross-sectional view of an IPM machine in accordancewith yet another embodiment of the invention.

FIG. 7 is a cross-sectional view of a rotor assembly of an IPM machinein accordance with another exemplary embodiment of the invention.

FIG. 8 is a partial cross-sectional view of a rotor assembly of an IPMmachine in accordance with another exemplary embodiment of theinvention.

FIG. 9 is a flow chart representing steps in a method for assembling anIPM machine.

FIG. 10 is a flow chart representing steps in another method forassembling an IPM machine.

DETAILED DESCRIPTION

As discussed in detail below, embodiments of the invention are directedtowards an IPM machine and a method for providing the same. The IPMmachine includes a stack of laminations preferably shaped in a dovetailconfiguration for engagement with multiple permanent magnets, whichlaminations are mounted in multiple protrusions circumferentially arounda shaft in a rotor assembly. As used herein, the term ‘dovetail’ refersto fan-shaped features that form a tight interlocking joint. Inparticular, the present invention is directed to a high-speed IPMmachine with high power-density ranging between about 1.375 kWpeak/Kg toabout 1.57 kWpeak/Kg.

When introducing elements of various embodiments of the presentinvention, the articles “a,” “an,” “the,” and “said” are intended tomean that there are one or more of the elements. The terms “comprising,”“including,” and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements. Anyexamples of operating parameters are not exclusive of other parametersof the disclosed embodiments.

FIG. 1 illustrates a cross-sectional view of a rotor assembly 10 of anIPM machine. The rotor assembly 10 includes a rotor core 12. The rotorcore 12 is configured to rotate about a longitudinal axis 14. of the IPMmachine. The rotor assembly 10 further includes a shaft 16 with multipleprotrusions 18 around the rotor core 12. In the illustrated embodiment,the shaft 16 also includes multiple bottom features 20 alternativelyarranged relative to the multiple protrusions 18. In a particularembodiment, the bottom structures 20 are configured to provide passages22 for a cooling fluid (not shown) within the rotor core 12. In anon-limiting example, the cooling fluid through the passages 22 may beairflow or a coolant for reducing mechanical stresses and eddy currentlosses in the rotor assembly 10. The rotor assembly 10 also includesmultiple stacks of laminations 24 disposed on the protrusions 18 anddovetailed circumferentially around the shaft 16. In one embodiment, thestacks of laminations 24 and the protrusions 18 of the shaft 16 areconfigured to provide multiple shaft top features 26 with spaces 28 forallowing flow of a cooling fluid. In other embodiments, the shaft topfeatures 26 may reduce mechanical stresses and eddy current losses ofthe rotor assembly 10. Inclusion of the shaft top features 26 mayfurther result in overall mass reduction of the rotor assembly 10.

Furthermore, the rotor assembly 10 includes multiple permanent magnets30 disposed between the stacks of laminations 24 within the rotor core12. The permanent magnets 30 generate a magnetic field to be radiallydirected in an air gap between the rotor assembly 10 and a stator (FIG.2). The magnetic field generated by the permanent magnets 30 furtherinteracts with a stator magnetic field to produce a torque. Thepermanent magnets 30 provide an arrangement, which is a dovetail spokeconfiguration, particularly well suited for high-speed applications. Thedovetail spoke configuration provides for superior magneticflux-concentration effects, thereby enhancing the machine power density.

In one embodiment, the permanent magnets 30 may be made ofneodymium-boron-iron. In another embodiment, the permanent magnets 30are made of samarium-cobalt, or ferrite, or Alnico. The rotor assembly10 also includes multiple bottom wedges 32 disposed on the bottomstructures 20 of the shaft 16. The multiple bottom wedges 32 arearranged to hold the multiple stacks of laminations 24 and the multiplepermanent magnets 30 in the rotor assembly and further prevents fromsplaying during high-speed rotations. In one embodiment, the bottomwedges 32 are non-metallic wedges for reducing the eddy current lossesin the rotor core 12. In a particular embodiment, the rotor assembly 10is also provided with multiple top wedges 34 covering the outerperiphery of the rotor assembly 10 to prevent the permanent magnets 30from splaying due to a centrifugal force generated in the high-speedrotor core 12.

Furthermore, the rotor assembly 10 also includes a stationary tube 35arranged co-axially in the center of the rotor core 12. The innersurface 36 of the shaft 16 and an outer surface 37 of the stationarytube 35 provide a rotor inner bore 38 for out flow of cooling fluid. Inone embodiment, the stationary tube 35 is a hollow tube with a centrebore 39 for allowing inflow of cooling fluid.

FIG. 2 is a cross-sectional view of a stator assembly 40 of the IPMmachine in FIG. 1 with a stator core 42. As illustrated herein, thestator core includes multiple segmented stator structures 44 andmultiple stator teeth 46 arranged circumferentially forming a cavity 48at a center 50 of the stator core 42. The stator assembly 40 generates amagnetic field and extends along the longitudinal axis 14 (shown inFIG. 1) with an inner surface 52 defining the cavity 48. The rotorassembly 10, as discussed above, is disposed within the cavity 48defined by the stator core 42. The stator assembly 40 includes multiplestator-slots 54 for concentrated windings 56 in between multiplesegmented structures 44, wherein each coil is wound around one statortooth. In one embodiment, the concentrated windings 56 include coppercoils. In another embodiment, the stator assembly includesfractional-slot concentrated windings. The stator assembly 40 alsoincludes multiple stator back irons 58.

In a particular embodiment, the multiple segmented structures 44 enablesincreased accessibility to a stator-slot area allowing a higher numberof concentrated windings 56 of copper around the stator teeth 46compared to a traditional stator of an IPM machine. This improves thepower density of the IPM machine. Furthermore, in one embodiment, thestator assembly 40 includes a coil wound around one stator toothindicating reduced utilization of windings in the end regions of each ofthe stator teeth 46 compared to a traditional IPM machine. This furtherimproves power density and efficiency.

FIG. 3 is a semi-cross-sectional view of the IPM machine 60 (FIG. 1 andFIG. 2) illustrating an exemplary distribution of magnetic flux lines 62in accordance with an embodiment of the invention. In the illustratedembodiment, magnetic flux lines 62 have a continuous flow from a rotor64 to a stator 66 region. As illustrated herein, a dovetail spokeconfiguration of a stack of laminations 68 and permanent magnets 70provides for superior magnetic flux-concentration effects, therebyenhancing the machine power density of the IPM machine 60. The bottomwedges 72 are preferably non-metallic and do not allow the flow ofmagnetic flux lines through them, thereby preventing magnetic fluxleakages. Similarly, the top wedges 73 are also preferably non-metallicand prevents the flow of magnetic flux lines 62 through them, therebyavoiding magnetic flux leakages. The flow of magnetic flux lines 62 (asshown in FIG. 3) through various areas of the IPM cross-section arebased on Finite Element Analysis (FEA). In one embodiment, multiple topwedges 73 and multiple bottom wedges 72 are provided on the laterallyopposite ends of multiple permanent magnets 70. The non-metallic topwedges 73 and bottom wedges 72 serve to obstruct the flow of magneticflux lines 62 and thereby assure that the magnetic flux lines 62 exitsfrom the permanent magnets 70 and couple with the stator windingssubstantially without leakage.

FIG. 4 is a partial cross-sectional view of an exemplary rotor assembly80. The rotor assembly 80 includes multiple stacks of laminations 82disposed on the protrusions 83 of the shaft 84. As illustrated herein,the stacks of laminations 82 are dovetailed circumferentially around theshaft 84. The dovetailed recess 86 between the stacks of laminations 82receives multiple permanent magnets 88, thereby, enabling the permanentmagnets 88 to be held radially between the stacks against centrifugalforce 90 caused by rotation. Further, in a particular embodiment, therotor assembly 80 is provided with multiple top wedges 92 covering theouter periphery of the rotor assembly 80 to positively restrain thepermanent magnets 88. Furthermore, each lamination is provided withlip-shaped features 85 which partially covers the top wedges 92. Thisalso ensures the permanent magnets 88 to be firmly held in the dovetailrecess 86 of the rotor assembly 80.

In one embodiment, the rotor assembly 80 is provided with multiplebottom wedges 94 disposed on multiple bottom structures 96 of the shaft84. In a preferred embodiment, the bottom wedges 94 are non-metallic andprevent magnetic flux leakages, thereby further reducing eddy currentlosses. In yet another embodiment, the bottom wedges 94 and the bottomstructures 96 are configured such that a portion of the bottom wedges 94lies in the dovetailed recess between the stacks of laminations 82beneath the multiple permanent magnets 88. This prevents splaying of themultiple stacks of laminations 82 which is a consequence of thecentrifugal forces acting upon the inclined faces of the “dovetail”feature in the laminations 82 and protrusions 83. Earlier designs haverelied on the permanent magnets 88 to prevent the splaying. However,since the permanent magnets 88 have a tendency to fracture, thepermanent magnets 88 do not restrain the splaying reliably and resultsin high mechanical stresses in the laminations 82.

FIG. 5 illustrates another exemplary embodiment of an IPM machine 100.The IPM machine 100 includes a stator assembly 102 and a stator core104. The stator core 104 also includes multiple stator teeth 106. Thestator assembly 102 is further configured with stator windings togenerate a magnetic field when excited with alternating currents andextends along a longitudinal axis 112 with an inner surface defining acavity. The IPM machine also includes a rotor assembly 108 and a rotorcore 110. The rotor core 110 is disposed inside the cavity formed by thestator core 104. The rotor assembly 108 is configured to rotate aboutthe longitudinal axis 112. The rotor assembly 108 includes multiplepermanent magnets 114 disposed between the stacks, which permanentmagnets 114 are configured to generate magnetic field. This magneticfield interacts with the stator magnetic field to produce torque. Therotor assembly 108 further includes a shaft 116 with multipleprotrusions 118. The rotor assembly 108 also includes multiple stacks oflaminations 120 disposed on the protrusions 118 and dovetailedcircumferentially around the shaft 116. The stacks of laminations 120are disposed around the shaft 116 in a reverse manner compared to theprevious embodiment. The laminations 120 are dovetailed around the shaft116 between the protrusions 118 providing for a lower radii shaftcompared to that of the previous embodiment. The lower radii shaft alsohave lesser volume and mass, as compared to that of previous embodiment,which substantially adds to the lightweight characteristic of the IPMmachine 100. Further, since the laminations 120 are dovetailed aroundthe shaft 116 between the protrusions 118, the splaying of laminationsdoes not exist in this embodiment and hence lowers the stresses in thelaminations 120.

The stacks of laminations 120 as shown in FIG. 5 further includemultiple magnetic regions 121 and non-magnetic regions 122. The lowerportions of the laminations 120 are heat treated for impartingnon-magnetic characteristics, thereby forming the non-magnetic regions122. Further, the non-magnetic regions 122 are dovetailedcircumferentially around the shaft in a reverse manner as compared tothe previous embodiment. Thus, the magnetic flux leakages in thenon-magnetic region 122 are substantially reduced.

FIG. 6 illustrates a semi-cross-sectional view of another exemplary IPMmachine 200. The IPM machine 200 includes a stator assembly 202 and astator core 204. The stator core 204 also includes multiple stator teeth206. The stator assembly 202 is further configured with stator windingsto generate a magnetic field when excited with alternating currents andextends along a longitudinal axis 208 with an inner surface defining acavity. The IPM machine 200 also includes a rotor assembly 210 and arotor core 212. The rotor core 212 is disposed inside the cavity andconfigured to rotate about the longitudinal axis 208. The rotor assembly210 further includes a shaft 214 with multiple protrusions 216. Therotor assembly 210 also includes multiple stacks of laminations 218disposed on the protrusions 216 and dovetailed circumferentially aroundthe shaft 214. In one particular embodiment, the laminations 218 may beheat-treated to provide continuous laminations of a bi-state magneticmaterial. Non-limiting examples of the bi-state magnetic materialinclude dual phase ferromagnetic material with a composition of Iron(Fe), 17.5% Chromium (Cr), 2% Nickel (Ni), 0.8% Aluminum (Al), 0.5%Carbon (C). In other non-limiting example of dual phase ferromagneticmaterial, Cobalt is added to increase the magnetization. In yet anothernon-limiting example of dual phase ferromagnetic material, chromium maybe replaced by weaker carbide forms, such as Mn. This increases themagnetization and further reduces the thermal gradient required tocreate the dual-phase structure. In another embodiment, the laminated218 are subjected to a localized surface treatment to form thenon-ferromagnetic regions. The rotor assembly 210 includes multiplepermanent magnets 220 disposed between the stacks of laminations 218,which permanent magnets 220 are configured to generate a magnetic fieldthat interacts with the stator magnetic field to produce a torque. In aparticular embodiment, the rotor assembly 210 includes a stack ofcontinuous laminations 218 of bi-state magnetic materials. Multiplelocations 222 of the laminations 218 (as shown in FIG. 6) cover theouter periphery of the rotor assembly 210 to restrain the multiplepermanent magnets 220 against the centrifugal force. Further, themultiple locations 222 are made non-magnetic by heat-treating to reducethe magnetic flux leakages. The non-magnetic regions obstruct the flowof magnetic flux and thereby assure that the magnetic flux exits fromthe permanent magnets 220 without leakage.

FIG. 7 illustrates a cross-sectional view of a rotor assembly 300 of yetanother exemplary IPM machine. The rotor assembly 300 includes a rotorcore 302. The rotor core 302 is configured to rotate about alongitudinal axis 303 of the IPM machine. The rotor assembly 300 furtherincludes a shaft 304 with multiple protrusions 305 around the rotor core302. In the illustrated embodiment, the shaft 304 also includes multiplebottom features 306 alternatively arranged relative to the multipleprotrusions 305. In a particular embodiment, the bottom structures 306are configured to provide passages 307 for a cooling fluid (not shown)within the rotor core 302. In another embodiment, the bottom structures306 are configured to provide spaces for airflow or a coolant. In anon-limiting example, the cooling fluid through the passages 307 may beairflow or a coolant for reducing mechanical stresses and eddy currentlosses in the rotor assembly 300. The rotor assembly 300 also includesmultiple stacks of laminations 308 disposed on the protrusions 305 anddovetailed circumferentially around the shaft 304. In one embodiment,the stacks of laminations 308 and the protrusions 305 of the shaft 304are configured to provide multiple shaft top features 310 with spaces orpassages for allowing flow of a cooling fluid. In other embodiments, theshaft top features 310 may reduce mechanical stresses and eddy currentlosses of the rotor assembly 300. The shaft top features 310 also resultin overall mass reduction of the rotor assembly 300. Further, the rotorassembly 300 includes multiple pair of permanent magnets 312 disposedbetween the stacks of laminations 308 within the rotor core 302. Thepair of permanent magnets 312 are further separated by multiple middlewedges 314 mounted between each pair of permanent magnets 312. Theinclusion of middle wedges 314 provides robustness to the rotor assembly300 and further provides substantial strength to the permanent magnets312. In one embodiment, the rotor assembly 300 may also be provided withmultiple top wedges 316 covering the outer periphery of the rotorassembly 300 to restrain the permanent magnets 312 against centrifugalforces.

The pair of permanent magnets 312 is configured to generate a magnetictorque by providing a magnetic field that interacts with a statormagnetic field and is radially directed in an air gap between the rotorand the stator. The pair of permanent magnets 312 provides anarrangement, which is a dovetail spoke configuration, particularly wellsuited for high-speed applications. The dovetail spoke configurationprovides for superior magnetic flux-concentration effects, therebyenhancing the machine power density. In one embodiment, the permanentmagnets 312 may be made of neodymium-boron-iron. In another embodiment,the permanent magnets 312 may be made of samarium-cobalt, or ferrite, orAlnico.

FIG. 8 is a partial cross-sectional view of the rotor assembly 300 inFIG. 7. The rotor assembly 300 includes multiple stacks of laminations402 disposed on the protrusions 403 of the shaft 404. As illustratedherein, the stacks of laminations 402 are dovetailed circumferentiallyaround the shaft 404. The dovetailed recess 405 between the stacks oflaminations 402 receives multiple pair of permanent magnets 406,thereby; enabling the pair of permanent magnets 406 to be held radiallybetween the stacks 402 against centrifugal forces 412. The pair ofpermanent magnets 406 are further separated by multiple middle wedges408 mounted between each pair of permanent magnets 406. The middlewedges 408 prevents splaying of the laminations 402 in the middleregion, thereby further preventing stresses in the laminations as wellon the permanent magnets 406. The inclusion of middle wedges 408provides robustness to the rotor assembly and further providessubstantial strength to the permanent magnets 406. Further, in aparticular embodiment, the rotor assembly 300 of FIG. 7 is provided withmultiple top wedges 410 covering the outer periphery of the rotorassembly 300 to restrain the permanent magnets 406 against thecentrifugal forces 412. Furthermore, each lamination is provided withlip-shaped features 407 which partially covers the top wedges 410. Thisalso ensures the permanent magnets 406 to be firmly held in the dovetailrecess 405 of the rotor assembly 300.

In another embodiment, the shaft 404 is provided with multiple bottomstructures 414 alternately arranged relative to multiple shaftprotrusions 403. The bottom structures 414 are arranged to substantiallyprevent splaying of the multiple stacks of laminations 402 underhigh-speed rotations. The centrifugal forces 412 acting upon theinclined faces of the “dovetail” feature in the laminations 402 andprotrusions 403, causes splaying of the laminations which issubstantially prevented by multiple edges of the bottom structures 414as shown in FIG. 8.

FIG. 9 is a flow chart representing steps in a method 500 for assemblingan IPM machine. The method 500 includes providing a stator assemblyincluding a stator core with multiple stator teeth in step 502. Thestator assembly extends along a longitudinal axis with an inner surfacedefining a cavity. The stator assembly is further configured with statorwindings to generate a stator magnetic field when excited withalternating currents. At step 504, the method 500 includes providing arotor assembly with a rotor core including multiple bottom wedgesdisposed on multiple bottom structures of a rotor shaft and isconfigured for holding multiple stacks of laminations and permanentmagnets. The rotor core of the IPM machine is disposed inside the cavityformed by the stator core and is configured to rotate about thelongitudinal axis. The rotor shaft includes multiple protrusionsalternately arranged relative to the bottom structures around the shaft.The stacks of laminations are disposed on the protrusions and dovetailedcircumferentially around the shaft. Furthermore, the multiple permanentmagnets are disposed between the stacks and configured to generate amagnetic field, which magnetic field interacts with the stator magneticfield to produce a torque. In a particular embodiment, the method alsoincludes providing multiple top wedges mounted on the multiple permanentmagnets for containing the multiple permanent magnets.

FIG. 10 is a flow chart representing steps in another method 600 forassembling an IPM machine. The method 600 includes providing a statorassembly including a stator core with multiple stator teeth in step 602.The stator assembly extends along a longitudinal axis with an innersurface defining a cavity. The stator assembly is further configuredwith stator windings to generate a magnetic field when excited withalternating currents. At step 604, the method 600 includes providing arotor assembly with a rotor core including multiple pair of permanentmagnets and middle wedges between each pair of permanent magnets. Thepermanent magnets are configured to generate a magnetic field, whichinteracts with the stator magnetic field to produce a torque. The rotorcore of the IPM machine is disposed inside the cavity formed by thestator core and is configured to rotate about the longitudinal axis. Therotor shaft includes multiple protrusions alternately arranged relativeto the bottom structures around the shaft. The stacks of laminations aredisposed on the protrusions and dovetailed circumferentially around theshaft. Furthermore, the multiple permanent magnets are disposed betweenthe stacks and configured to generate a magnetic field that interactswith the stator magnetic field to produce torque. In a particularembodiment, the method also includes providing multiple top wedgesmounted on the multiple permanent magnets for containing the multiplepermanent magnets.

Advantageously, the various embodiments of the invention decouple theelectromagnetic and mechanical requirements in an IPM machine byeliminating the need for bridges and center posts. This substantiallyincreases the machine power density. Also the spoke configuration hasflux-concentration effects, which further increases the machine powerdensity. The dovetailing of the rotor laminations into the shaft allowsgoing to higher tip speeds (rotor outer radius speed), which furtherincreases the machine power density. The lower radii shaft in oneembodiment provides for a shaft with less mass and further adding to alightweight characteristic of the IPM machine. Other advantages of theinvention include the reduction of the magnetic flux due to the bi-statelaminations and non-metallic or non-magnetic wedges. The rotor assemblyand the various associated components are primarily configured toprovide maximum power density and minimum eddy current losses.Furthermore, the present invention provides additional advantages interms of low volume, mass and cost. These techniques and systems thus,allow for highly efficient permanent magnet machines.

The IPM machines, as described above, may be well suited for hybridapplications. However, such IPM machines may be employed in a variety ofother applications. One of them includes aviation applications, such asin aircraft engines. The PM machines can also be used for othernon-limiting examples such as traction applications, wind and gasturbines, starter-generators for aerospace applications, industrialapplications and appliances.

Of course, it is to be understood that not necessarily all such objectsor advantages described above may be achieved in accordance with anyparticular embodiment. Thus, for example, those skilled in the art willrecognize that the systems and techniques described herein may beembodied or carried out in a manner that achieves or optimizes oneadvantage or group of advantages as taught herein without necessarilyachieving other objects or advantages as may be taught or suggestedherein.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. An internal permanent magnet machine comprising: a stator assemblycomprising a stator core, the stator core comprising a plurality ofstator teeth, the stator assembly is further configured with statorwindings to generate a stator magnetic field when excited withalternating currents and extends along a longitudinal axis with an innersurface defining a cavity; and a rotor assembly comprising a rotor core,the rotor assembly disposed inside said cavity and configured to rotateabout the longitudinal axis, wherein the rotor assembly comprises: ashaft comprising a plurality of protrusions alternately arrangedrelative to a plurality of bottom structures; a plurality of stacks oflaminations, the stacks disposed radially outward on the protrusions anddovetailed circumferentially around the shaft; a plurality of permanentmagnets disposed between the stacks and are configured to generate amagnetic field, which magnetic field interacts with the stator magneticfield to produce a torque; and a plurality of bottom wedgessubstantially distinct from the shaft and disposed radially outward onthe bottom structures of the shaft and configured to hold the pluralityof stacks to the shaft.
 2. The machine according to claim 1, furthercomprising a plurality of top wedges mounted on the plurality ofpermanent magnets for containing the plurality of permanent magnets. 3.The machine according to claim 2, wherein the plurality of top wedgescomprise non-metallic wedges.
 4. The machine according to claim 2,wherein the laminations comprises lip-shaped features configured to beseated partially on the top wedges.
 5. The machine according to claim 1,wherein the plurality of bottom wedges comprise non-metallic wedges. 6.The machine according to claim 1, wherein the bottom structures of theshaft are configured to prevent splaying of the plurality of the stacksof laminations.
 7. The machine according to claim 1, wherein the bottomstructures are configured to reduce mechanical stress generated in themagnets and stacks of laminations.
 8. The machine according to claim 1,wherein the bottom structures are configured to reduce eddy currentlosses in the rotor assembly.
 9. The machine according to claim 1,wherein the bottom structures are configured to provide a passage for acooling fluid.
 10. The machine according to claim 1, wherein theplurality of protrusions and the plurality of stacks of laminations areconfigured to provide a passage for a cooling fluid.
 11. The machineaccording to claim 9, wherein the cooling fluid comprises ambient air ora coolant.
 12. The machine according to claim 1, further comprising astationary tube within the shaft, wherein a passage inside the tubeprovides for in flow of a coolant.
 13. The machine according to claim12, wherein a passage between the shaft and the tube provides for outflow of the coolant.
 14. An internal permanent magnet device comprising:a stator assembly comprising a stator core, the stator core comprising aplurality of stator teeth, the stator assembly is further configuredwith stator windings to generate a magnetic field when excited withalternating currents and extends along a longitudinal axis with an innersurface defining a cavity; and a rotor assembly comprising a rotor core,the rotor assembly disposed inside said cavity and configured to rotateabout the longitudinal axis, wherein the rotor assembly comprises: ashaft comprising a plurality of protrusions; a plurality of stacks oflaminations, the stacks mounted in between the protrusions anddovetailed circumferentially around the shaft, wherein the laminationscomprises a plurality of magnetic regions and a non-magnetic regions;and a plurality of permanent magnets disposed between the stacks and areconfigured to generate a magnetic field, which magnetic field interactswith the stator magnetic field to produce a torque.
 15. The machineaccording to claim 14, wherein a part of the plurality of the rotorlaminations dovetailed within a region of the shaft protrusions is heattreated for non-magnetism.
 16. The machine according to claim 14,further comprising a plurality of heat-treated non-magnetic bridges orwedges mounted on the plurality of permanent magnets for containing theplurality of permanent magnets.
 17. A method for assembling an internalpermanent magnet machine, the method comprising: providing a statorassembly comprising a stator core, the stator core comprising aplurality of stator teeth, the stator assembly is further configuredwith stator windings to generate a stator magnetic field when excitedwith alternating currents and extending along a longitudinal axis withan inner surface defining a cavity; and providing a rotor assemblycomprising a rotor core, the rotor assembly disposed inside said cavityand configured to rotate about the longitudinal axis, wherein the rotorassembly comprises: a shaft comprising a plurality of protrusionsalternately arranged relative to a plurality of bottom structures; aplurality of stacks of laminations, the stacks disposed radially outwardon the protrusions and dovetailed circumferentially around the shaft; aplurality of permanent magnets disposed between the stacks and areconfigured to generate a magnetic field, which magnetic filed interactswith the stator magnetic field to produce a torque; and a plurality ofbottom wedges substantially distinct from the shaft and disposed on thebottom structures of the shaft and configured to hold the plurality ofstacks and the plurality of permanent magnets; and a plurality of topwedges mounted on the plurality of permanent magnets and covering theouter periphery of the rotor assembly to positively restrain theplurality of permanent magnets.
 18. The method of claim 17, furthercomprising providing the stacks of laminations disposed in between theprotrusions of the shaft.